The present invention relates generally to digital communications systems, and more specifically, to an approach for processing data received from a communications channel to equalize and remove distortion and noise from the data.
There is a continuing need for higher performance digital data communications systems. Perhaps no where is this need more evident than on the worldwide packet data communications network now commonly referred to as the xe2x80x9cInternet.xe2x80x9d On the Internet, the xe2x80x9crichnessxe2x80x9d of content is constantly increasing, requiring an ever increasing amount of bandwidth to provide Internet content to users. As a result of this increased demand for bandwidth, significant efforts have been made to develop new types of high-speed digital data communications systems. For example, optical fiber based networks are being built in many large metropolitan areas and undersea to connect continents. As another example, new wireless protocols are being developed to provide Internet content to many different types of small, portable devices.
One of the significant drawbacks of deploying many of these new types of high-speed digital data communications systems is the high cost and amount of time required to develop and build out the new infrastructure required by the systems. Because of these high costs, many new high-speed digital data communications systems are initially deployed only in densely populated areas, where the cost of building out the new infrastructure can be quickly recovered. Less populated areas must often wait to receive the new communications systems and some rural areas never receive the new systems where it is not cost effective to build the infrastructure.
For several reasons, significant efforts are being made to utilize conventional twisted pair telephone lines to provide high-speed digital data transmission. First, a significant amount of twisted pair telephone line infrastructure already exists in many countries. Thus, using conventional twisted pair telephone lines avoids the cost of building expensive new infrastructure. Second, conventional twisted pair telephone lines extend into customers"" homes and businesses, avoiding the so-called xe2x80x9clast milexe2x80x9d problem. As a result of recent development efforts in this area, several new communications protocols, such as ADSL, G.Lite and VDSL, have been developed for providing high-speed digital transmission over conventional twisted pair telephone lines.
Despite the advantages to using conventional twisted pair telephone lines to provide high-speed digital communications, there are some problems with this approach. First, conventional twisted pair telephone lines cause signal attenuation per unit length that increases rapidly with frequency. A moderate length twisted pair line, for example around fifteen thousand feet, may cause only a few decibels (dB) of attenuation in the voice band, for which the line was originally designed, but many tens of dB of attenuation at higher transmission frequencies, for example around 1.1 MHz for ADSL. This results in a transfer function with a wide dynamic range, making channel equalization more difficult. The transfer function is further complicated by bridge taps and impedance mismatches between line sections that cause reflections and echoes at the receiver. Furthermore, the complexity of the transfer function is also increased by filtering performed at the transmitter and receiver.
The standards for ADSL and G.Lite specify Discrete Multitone (DMT) modulation. DMT is also under consideration for use in VDSL systems. DMT modulation generally involves transmitting digital data on a number of carriers simultaneously. Modulation and demodulation are performed using a Fast Fourier Transform (FFT). A cyclic prefix is introduced to ensure separation between successive DMT symbols and eliminate inter-symbol interference (ISI). In practice, the cyclic prefix is necessarily quite short, generally much shorter than the impulse response of the communications channel. This often results in significant ISI being present in the received data. Large amounts of ISI cause a large reduction in the available communications bandwidth.
Standard equalizers used in digital communication systems, such as adaptive LMS and RLS equalizers, are generally inappropriate for DMT systems since they are not designed to eliminate ISI. The current state of the art in equalizer design has the objective of shortening the overall channel plus equalizer impulse response so that the overall response is shorter than the cyclic prefix length. Various attempts to meet this requirement have been made. See for example, Optimal Finite-Length Equalization for Multicarrier Transceivers, by N. Al-Dhahir and J. M. Cioffi, IEEE Transactions on Communications, pages 56-63, January 1996; and A Multicarrier Primer, by J. M. Cioffi. Determining equalizer coefficients is generally a computationally inefficient process and can be quite sensitive to noise, which limits the practical application of these techniques.
In addition to the equalization problem, twisted pair lines suffer from various forms of interference. Up to fifty twisted pairs are conventionally grouped together in binders. As a result, a signal on one pair can cause interference on other pairs in the same binder. This interference is called crosstalk and results in a reduced signal-to-noise ratio (SNR) at the receiver. Current approaches to mitigate crosstalk require access to the signal transmitted on the interfering line. This makes current approaches useful only in a central office environment, where the signals on all pairs in a binder are available. Thus, none of the existing crosstalk mitigation approaches are suitable when only the received signal is available.
Based on the foregoing, there is a need for an approach for processing data received from a communications channel that does not suffer from the limitations of conventional approaches. There is a particular need for an approach for processing data received from a communications channel that provides communications channel equalization and interference mitigation.
According to another aspect of the invention, a method is provided for processing data received from a communications channel. According to the method, received data is received from the communications channel, wherein the received data is based upon both modulated data and distortion introduced by the communications channel, wherein the modulated data is the result of original data modulated onto one or more carriers. The method also includes generating sampled data by sampling the received data at a specified rate that satisfies specified sampling criteria, generating a filtered observation sequence by processing the sampled data and generating estimated modulated data by processing the filtered observation sequence using a recursive filter. Finally, an estimate of the original data is recovered by demodulating the estimated modulated data.
According to another aspect of the invention, an apparatus is provided for processing data received from a communications channel. The apparatus includes an analog-to-digital converter configured to generate sampled data by sampling, at a specified rate that satisfies specified sampling criteria, received data received from the communications channel, wherein the received data is based upon both modulated data and distortion introduced by the communications channel, and wherein the modulated data is the result of original data modulated onto one or more carriers. The apparatus also includes a first filter mechanism configured to generate a filtered observation sequence by processing the sampled data and a second filter mechanism configured to generate estimated modulated data by processing the filtered observation sequence. The apparatus also includes a demodulator configured to recover an estimate of the original data by demodulating the estimated modulated data. The apparatus may also include an impulse response shortening filter before the demodulator, depending upon the requirements of a particular application.